This invention concerns improvements relating to electrical surge arresters, also known as surge diverters, which, as is well known, are used in high voltage electric installations for providing a path to ground for surge overvoltages occasioned for example by lightning strikes and as switching transients.
In GB-A-2188199 there is described a surge arrester which is primarily intended for use in distribution class applications, but can be coupled with other electrically matched arresters in a series parallel arrangement for higher voltage station class applications as described in GB-A-2230661. This surge arrester comprises a rigid core of great physical strength upon which there is provided a shedded outer housing of polymeric material, the core consisting of a plurality of ceramic varistor blocks stacked end to end and encased within a shield of reinforced rigid plastics material. The reinforced rigid plastics material shield is preferably bonded directly to the external surface of the varistor block stack, though proposals have been made to provide a thin spacing layer of Mylar(trademark) between the varistor blocks and the shield. There may be a number of metal (eg aluminium) spacer blocks incorporated into the varistor block stack for the purpose of providing the stack with an overall length sufficient to avoid flashover externally of the arrester and terminal blocks are provided at opposite ends of the stack for enabling the coupling of the arrester to associated equipment.
As described in GB-A-2188199 the rigid core and polymeric housing of the surge arrester provides significant advantages as compared to porcelain housed surge arresters which have long suffered a fragility problem. Conventional porcelain housed surge arresters commonly have an inert gas filling which can literally blow the arrester housing apart if the arrester is subjected to an excessive overvoltage, littering the surrounding area with hot fragments and causing fires. Pressure relief rupturable diaphragms can be provided in the end cap arrangements of gas filled porcelain housed surge arresters, but these provisions complicate the arrester construction and the fragility problem of the porcelain housing still remains.
The rigid cored, polymer housed, gapless surge arrester of GB-A-2188199 is manufactured to avoid gaseous inclusions and retains its structural integrity even under failure. Additionally, it enables support insulators to be dispensed with, since the rigid core of the arrester is sufficiently strong to support the loading which conventionally is supported by the provision of support insulators. The inherent strength of the arrester of GB-A-2188199, which results from its rigid core and gapless construction, enables it to be coupled into a series parallel array capable of handling station class voltages as described in GB-A-2230661. As a result of these and other advantages, the surge arrester of GB-A-2188199 has enjoyed considerable commercial success.
Notwithstanding the success of the surge arresters of GB-A-2188199 and GB-A-2230661 we have, as a part of our continuous improvement strategy, considered how component costs, assembly times and manufacturing process times might be reduced. The manufacture of surge arresters in accordance with GB-A2188199 requires curing processes to cure the reinforced epoxy material that is applied to the varistor block stack in an uncured state and, if a silver loaded adhesive is provided between the contiguous faces of the stacked blocks to improve electrical contact therebetween, additionally requires a curing process for the adhesive. After curing, the core assembly is then sleeved with its polymeric housing in a separate operation and finally end caps are fitted. The manufacturing time required for these processes is considerable and demands the availability of curing ovens, and a manufacturing technique which could eliminate the curing process would show considerable benefit in manufacturing space and time. Any reduction in component costs would be an added bonus.
It is, accordingly, the object of the present invention to provide an electrical surge arrester which can be more readily manufactured than the surge arrester of GB-A-2188199 without prejudice to the advantages demonstrated by that surge arrester.
The present invention stems primarily from the realization that the external passivation coating that is invariably provided by manufacturers of metal oxide varistor blocks on all surfaces of the blocks other than their metallized contact surfaces can in fact be dispensed with, thereby leading to a reduction in component costs. Varistor blocks conventionally are solid circularly cylindrical in shape with metallized contact coatings on their circular axially end faces and passivation coatings on their cylindrical external surfaces, the passivation coating comprising an epoxy resin or glass or a separately fitted electrically insulating collar. The passivation coating enables a varistor to be used to its full electrical capability by avoiding external flashover problems between the contact surfaces, and its provision is also useful to the manufacturer in that it enables classification and quality control operations to be conducted. However, the provision of the passivation coating significantly contributes to the manufacturing cost of varistor blocks.
If the need for a passivation coating can be dispensed with, then the limitations that the need for a passivation coating have imposed upon the shapes of varistor blocks can also be dispensed with. The process by which passivation coatings are most economically provided on varistor blocks is to spray an epoxy resin or glass material on to the surface of the varistor block and then to bake the varistor in an oven until the resin is cured on to the varistor surface or until the glass material has sintered on to the varistor body. This process has hindered the production hitherto of varistor blocks with through holes, since the manufacture of a varistor block with a through hole and with an effective passivation coating on the surface of the through hole would impose additional difficulties for the varistor block manufacturer.
By dispensing with the passivation coating, varistor blocks with through holes, for example an axial through hole, can be manufactured at reasonable cost. A stack of such varistor blocks can then be retained in face-to-face contact between end terminations by means of an electrically-insulating rod appropriately secured to the end terminations and the present invention proposes to take advantage of such an arrangement. A surge arrester comprising varistor blocks with axial through holes which are retained in a stacked configuration and supported by an electrically insulating rod has previously been proposed in GB-A-2073965 but has not been manufactured. Other surge arresters wherein varistor blocks have a supportive rod extending through holes in the blocks are proposed in U.S. Pat. No. 4,262,318, EP-A-0141239, U.S. Pat. No. 4,825,188 and WO-A-95/10846. The surge arresters of U.S. Pat. No. 4,262,318 and EP-A-0 141 239 are of the previously mentioned porcelain housed type wherein the physical strength of the arrester, such as it is, is provided by the porcelain housing. The surge arrester of U.S. Pat. No. 4,825,188 has a similar construction to a surge arrester embodying to the present invention as hereinafter described, but U.S. Pat. No. 4,825,188 does not disclose or suggest the present invention. Likewise WO-A-95/10846 discloses a similar surge arrester but does not disclose or suggest the present invention.
In accordance with the present invention, a stack of non-passivated varistor elements is retained in face-to-face contact between end terminations by means of an electrically insulating rod passed through through-holes in the varistor elements and secured to the end terminations, the through-holes and the rod are sized such that there is a clearance between the surface of the rod and the surrounding surface of the through-holes, and a silicone rubber material fills the void that otherwise would exist between the rod and the varistor blocks and extends around the external surfaces of the varistor blocks, the silicone rubber material serving as a passivation coating for the varistor blocks.
The silicone rubber material can be injected in liquid form and will cure into a solid without external intervention. The external configuration of the silicone rubber on the outside of the arrester can advantageously include integral sheds to increase the external tracking distance of the arrester between the end terminations and to serve a weather shedding function. Alternatively the silicone rubber on the outside of the arrester could have a circular cylindrical surface and a separate shedded outer housing, formed for example of heat shrink polymeric material or of a mechanically released elastomeric material or of an in situ moulded plastics material, could be provided on the arrester, though this is not preferred in view of the additional manufacturing steps that would be required.
The invention also extends to a method of manufacturing a surge arrester comprising securing a plurality of non-passivated varistor blocks between end terminations by passing an electrically insulating rod through aligned through-holes in the varistor blocks and securing the rod to the end terminations, the through-holes in the varistor blocks being larger than the rod cross-section, injecting a liquid silicone rubber material into the void between the rod and the surfaces of the through-holes in the varistor blocks and around the external surfaces of the stacked varistor blocks, the liquid silicone material being selected to serve a surface passivation function on the varistor blocks, and permitting the silicone rubber material to solidify.
The through-holes in the varistor blocks are preferably axial, since this permits the rod to locate centrally with respect to the stacked varistor blocks and enables a screw-threaded connection of the ends of the rod to the terminations to be utilized. It is however not inconceivable that a crimp type or lock screw type of attachment of the terminations to the through rod might be utilized. The rod is preferably axial, but could be off axis, and also there might be more than one such rod. For simplicity, however, a central, axial rod making screw-threaded or crimped attachment to the arrester terminations is preferred.
By virtue of the use of non-passivated varistor blocks a reduction is achieved in the arrester component costs. By use of appropriate assembly jigs, the varistor blocks can readily be threaded onto their central rod and secured between end terminations screw-threaded and/or compressed onto the ends of the rod. The sub-assembly thus formed can then be placed in an appropriate mould for the injection of the silicone rubber material and a finished surge arrester comes out of the mould. Production costs can thereby be lowered as compared to production costs for the surge arrester described in GB-A-2188199.
The support rod can advantageously be tensioned in the arrester construction, for example by appropriate adjustment of screw-threaded end terminations and/or by inclusion of a pre-load means (one or more Belleville washers, for example) in the varistor stack between the end terminals. This pre-loading assists the surge arrester in withstanding operational mechanical loading.
Tests that we have conducted have established that suitable electrical grade silicone rubber material, for example SILOPREN(trademark) LSR available from Bayer, satisfactorily effects passivation of the varistor block surfaces. EPDM, disclosed in U.S. Pat. No. 4,825,188, is not a suitable material for this purpose.
The above and further features of the present invention are set forth in the appended claims and will be made clear from consideration of the following detailed description given with reference to the accompanying drawings.